In air turbidity studies, it has been found that the brightness of the sky near the sun is generally well correlated to aerosol loading of the sky. In particular, sky radiation intensity at angular distances of less than 10.degree. from a limb of the sun can be utilized for meteorological purposes to deduce information about the size distribution of atmospheric aerosols, as well as for astronomical purposes to derive an indication of the coronagraphic quality of the sky. By measuring sky intensities at angular distances less than 10.degree. from the sun to an accuracy of at least 5 percent, it is possible to determine certain aspects of atmospheric particulates that affect the radiation heat budget.
A coronametric instrument to measure near-sun sky brightness was described by J. W. Evans in the Journal of The Optical Society of America, Vol. 38, No. 12, page 1083, 1948. In the instrument reported by Evans, a single optical axis is adapted to be aligned with the sun. On the optical axis, there is provided an annular entrance aperture including a first occulting disc which prevents direct rays from the sun from falling on an entrance aperture; light rays from the sky surrounding the sun enter the annular entrance aperture. The ray paths intersect an uncoated mirror that projects light to the back face of a second occulting disc. Light in the second ray path is projected through an optical wedge to the front face and the second occulting disc. The optical density wedge is adjusted so that the intensity of light projected on the front face and past the back face in the two ray paths appears equal to a viewer. Hence, it is necessary to adjust the optical wedge until the light in the two ray paths appear to have equal brightness, an operation which is time consuming and can result in inaccuracies. The prior art instrument is subject to inaccuracies as great as .+-. 30 percent because of the inability of the eye to discern differences in the brightness of the light in the two ray paths illuminating the second occulting disc. It has also been found desirable in measuring aerosol loading of the sky near the sun to make measurements at two or more different angular positions in proximity to the sun. In the Evans instrument, this is not possible because the light paths emanate from a single field of view.